A variety of capacitive pressure sensor types are known. These include a two-plate type that utilizes at least one flexible diaphragm as a plate of a parallel plate capacitor. The other plate may also be flexible, but most devices have a rigid second plate separated from the flexible diaphragm by a narrow space. The separation between the plates (with the diaphragm unflexed) is usually determined by the height of a wall between the diaphragm and the rigid plate which serves both to separate the plates and to enclose the space between them. It is necessary to seal the enclosed space, usually in vacuum, in order to provide a reference pressure on one side of the flexible diaphragm.
A specific example of the two-plate flexible diaphragm type is disclosed in U.S. Pat. No. 4,178,621, "Electromechanical Pressure Transducer" granted to Simonelic and assigned to Motorola, Inc. The diaphragm and the substrate (for mounting the rigid plate)in the Simonelic device are separated by a wall consisting of an annular ring of insulating glass around the periphery of the device. Simonelic further discloses the use of a relatively thin ceramic disc and a relatively thick ceramic base for use, respectively, as a diaphragm and a substrate. Both the ceramic disc and the ceramic base have metallic surfaces deposited thereon for use as capacitive plates. Each metallic surface has a projecting conductive "finger" associated therewith for external electrical connection to a circuit which senses changes in the capacitance between the plates. The rigid plate's "finger" extends radially from the central plate area and passes under the glass ring, and the flexible plate's "finger" similarly passes over the glass ring. The device is sealed by subjecting the assembly to a high temperature, at which point the glass ring softens, thereby bonding the diaphragm to the substrate.
The sealing method employed for such a device is more fully disclosed in column 4, lines 58-61 of U.S. Pat. No. 4,345,299, "Capacitive Pressure Transducer Assembly With Improved Output Lead Design", granted to Ho and also assigned to Motorola, Inc. The glass ring is a silk screened "thick film paste comprising glass frit particles" which is "fired" at high temperature to join the diaphragm to the base substrate.
A similar sealing method is disclosed in even more detail in U.S. Pat. No. 4,184,189, "Capacitive Pressure Sensor and Method of Making It", granted to Davis and also assigned to Motorola, Inc. The Davis patent discloses a glass diaphragm and a glass substrate joined by a glass ring. Sifted ground glass is "mixed into a paste for screen printing. The paste is prepared by mixing the ground glass with a binder and an organic solvent . . . [which is] then screen printed on the base glass . . . [dried] . . . and heated . . . to remove the binder." (See column 4, line 58 to column 5, line 6). After evacuating the furnace, heating and cooling the device, mounting the diaphragm on shims that determine the spacing between the diaphragm and the base (about 0.013 to 0.051 mm), a 75 gram weight is placed on top of the assembly which is placed in an evacuated (about 100 mm Hg) furnace and is "heated to 500.degree. C. for thirty minutes during which time the sealing glass mixture flows and seals together the base and diaphragm." (See column 5, lines 18-21).
The method of providing plate electrodes which emerge radially as "fingers" from the central plate areas under, over, or through the glass frit seal is a simple way of "getting at" the otherwise inaccessible capacitor plates (within the evacuated chamber) for electrical connection to external circuitry for measurement of capacitive changes. The method is well-known, as may be seen in various other U.S. Pat. Nos., including: U.S. Pat. No. 4,287,553 "Capacitive Pressure Transducer", granted to Braunlich, U.S. Pat. No. 4,292,659, "Pressure Sensing Capacitive Transducer", granted to Lao, both assigned to The Bendix Corporation; U.S. Pat. No. 4,207,604, "Capacitive Pressure Transducer With Cut-Out Conductive Plate", and U.S. Pat. No. 4,388,668, "Capacitive Pressure Transducer", both granted to Bell et al, and both assigned to Kavlico Corporation.
A limitation of the glass frit sealing method used in all of the above U.S. patents is that it can only be used for sealing relatively large devices, i.e. devices having a diameter of about 2-3 centimeters or more (the capacitive devices are usually planar capacitors, typically having a right circular cylindrical shape with a radius much larger than its height). The minimum capacitive gap distance achievable with the glass frit sealing method is not particularly narrow. Since the closer the plates are, the higher is the capacitance therebetween, the lack of narrowness in the gap must be compensated for (for a given capacitive requirement) by increasing the area of the capacitor plates (the capacitance of a parallel plate capacitor is directly proportional to the area of the plates and inversely proportional to the distance therebetween). Another limitation of the glass frit method is that the devices must usually be made one at a time.
A method for manufacturing a plurality of miniature (on the order of 5 mm) two-plate capacitive pressure transducers is disclosed in U.S. Pat. No. 4,261,086, "Method for Manufacturing Variable Capacitance Pressure Transducers", granted to Giachino et al, and assigned to Ford Motor Company. The method comprises the steps of (1) forming a plurality of spaced recesses (for use as capacitor plate diaphragms) in the surface of a semiconductor wafer, (2) doping the semiconductor wafer to enhance its electrical conductivity, (3) forming spaced areas of electrically conductive material on a surface of a dielectric material (for use as rigid capacitor plates), and (4) attaching the surface of the semiconductor wafer to the surface of the dielectric such that each of the recessed diaphragms covers one of the rigid plates in substantially parallel alignment. The detailed description in the patent discloses that the bonding of the wafer to the dielectric may be accomplished with the use of an anodic bonding technique (see column 6, lines 3-14).
In the Giachino device, access to the capacitor plates for external electrical connection is made through conductive holes formed in the dielectric. (See column 5, lines 15-68, and column 6, lines 1-3). The holes are made using ultrasonic, abrasive, or laser drilling techniques and are made conductive by depositing a metallo-organic solution in the holes and drying. The metallic salt residue left on the surfaces of the holes is heated, causing decomposition of the salts into a metal film that is electrically conductive. If desired, either one or both of the holes is sealed by melting solder which flows by capillary action into the holes. Solder "bumps" are then formed at the hole entrance for external electrical connection (see column 5, lines 26-46). The wafer is then cut to produce individual capacitive pressure transducer assemblies. See also related U.S. Pat. No. 4,277,814 "Semiconductor Variable Capacitance Pressure Transducer Assembly", granted to Giachino et al, and No. 4,386,453, "Method for Manufacturing Variable Capacitance Pressure Transducers", also granted to Giachino et al, both assigned to Ford Motor Company. A limitation in all of the Giachino patents is the relatively complex hole drilling, metal salting of the holes, and hole solder-sealing required to gain access to the capacitor plates for external electrical connection. The simpler method of using a projecting conductive "finger" is not used to gain access to the rigid plate inside the chamber as in the frit glass structures because the anodic bonding technique cannot be successfully used to bond metal to silicon.
Another miniature two-plate capacitive pressure sensor structure is disclosed in U.S. Pat. No. 4,405,970, "Silicon-Glass-Silicon Capacitive Pressure Transducer", granted to Swindal et al, and assigned to the owner of this patent application. In that patent, the techniques used in fabricating active semiconductor devices and in thin film technology are applied to fabricate a plurality of devices from a single substrate, each having a silicon diaphragm mounted over a silicon substrate and separated by a layer of glass. The diaphragm and substrate are doped to make them conductive. A layer of glass supports each diaphragm in parallel alignment over an associated glass or silicon pedestal (with or without metalization) formed on the silicon substrate. The pedestals are used to decrease the parasitic capacitance that would otherwise exist between and in the vicinity of the joined surfaces around the periphery of each device. These devices are particularly well suited for miniaturization because of the fine dimensional control achievable using the semiconductor and thin film technologies. However, despite the use of pedestals, moats, etc., parasitic capacitance remains to some degree. See also a related U.S. Pat. No. 4,415,948, "Electrostatic Bonded, Silicon Capacitive Pressure Transducer", granted to Grantham et al, also assigned to the owner of this patent application.
A miniature three-plate capacitive pressure sensor that helps solve the parasitic capacitance problem, described above in connection with the Swindal and Grantham two-plate sensors, is disclosed in pending U.S. patent application No. 527,531, owned by the owner of this patent application. In the three-plate device, several layers of a glass dielectric of borosilicate glass are deposited on a doped silicon substrate. In between the glass layers a central rigid conductive capacitor plate is deposited, along with a projecting conductive "finger" for external electrical connection. A doped silicon diaphragm is field assisted bonded in vacuum to a glass wall around the periphery of each device. The reason for using the several layers of glass dielectric between the silicon substrate and the silicon diaphragm is to create an additional parasitic capacitance between the silicon substrate and the central rigid plate and its associated "finger" electrode which is in series with the parasitic capacitance between the substrate and the diaphragm. By connecting a "Capacitive Pressure Transducer Signal Conditioning Circuit", as disclosed in pending U.S. patent application Ser. No. 527,530, also owned by the owner of this patent application, between the diaphragm, the substrate, and the central capacitor plate (by means of the "finger" electrode), the parasitic capacitances between the diaphragm and the substrate and between the central plate and the substrate may be cancelled electrically. The remaining variable capacitance between the diaphragm and the central plate is then substantially free of the parasitic capacitance masking effects. The three-plate device is also particularly advantageous because of its use of a projecting "finger" electrode which passes out from the central plate, through two of the glass layers, and out to the periphery of the device. Part of the diaphragm and several of the glass layers are etched away to expose the electrode's contact surface.